Effect of ibotenic acid on chloride permeability of insect muscle-fibres

Abstract

1. Ionic conductances ( G) of the resting membrane of the anterior coxal adductor muscle-fibre of the locust Schistocerca gregaria were determined by ion substitution. The following values were obtained: G K = 58.6 per cent and G C1 = 41.4 per cent in standard (chloride) saline, G CH3SO4 = 12.0 per cent in methylsulphate saline (all values expressed as percentages of the total membrane conductance in standard saline, assuming G Na = 0 and G SO4 = 0. 2. Ibotenic acid, a structural analogue of glutamic acid, increases the conductance of the coxal adductor muscle-fibre. The relative contributions of anions to the conductance increase induced by ibotenic acid were: Cl −,99.5 per cent in standard saline; CH 3SO 4 −, 5.6 per cent in methylsuphate saline, and SO 4 2−, < 0.5 per cent in sulphate saline. 3. Ibotenic acid has a negligible effect on cation conductance. 4. E IA and E GABA, the reversal potentials for the action of ibotenic acid and γ-aminobutyric acid respectively, were within 2 mV. of each other and 4 mV. of the resting potential, when microelectrodes filled with potassium sulphate were used for their measurement. This is to be expected if both approximate to the chloride equilibrium potential. 5. When electrodes filled with potassium methylsulphate were used, E IA was significantly less negative than E GABA. It seems likely, therefore, that ibotenic acid increases G CH3SO4, whereas γ-aminobutyric acid does not.